Separation membranes are utilized as filtration membranes in recent years in various fields, e.g., fields of water processing such as manufacture of drinking water, water purification treatment, or waste water treatment, and in a field of food industries. In a field of water processing such as manufacture of drinking water, water purification treatment, or waste water treatment, filtration with separation membranes has been used for removing impurities in water in place of conventional sand filtration and flocculation-precipitation processes. In food industries also, separation membranes are used for removing yeast utilized for fermentation, or for the concentration of liquids.
Polymer separation membranes are used in these various fields, and in a water treatment field such as water purification treatment or waste water treatment, it is required to further improve water permeating performance in filtration, since a great amount of water should be filtered. Greater water permeating performance makes it possible to reduce the area of membrane to be used for filtration process, and a resulting compact filtration apparatus can reduce the facility costs, so that it is also advantageous from the costs of membrane exchange and the space for installation.
In a field of water purification treatment, separation membranes are also required to have chemical resistance, since a sterilizer such as sodium hypochlorite is added to the water in a membrane module for the purpose of preventing biofouling on the membrane surface, or membranes themselves should be washed with an acid, an alkali, chlorine or a surfactant. Further, high physical strength is required of separation membranes so as not to cause breakage during use.
Thus, separation membranes are required to have superior separation performance, chemical strength (in particular, chemical resistance), physical strength and permeability. Accordingly, separation membranes made of polyvinylidene fluoride resins having both chemical strength (in particular, chemical resistance) and physical strength have been used in various fields.
Further, in the fields of manufacture of drinking water, pharmaceutical manufacture, and food industries, various sterilizing techniques are applied to manufacturing lines and products, since if pathogenic microbes such as a virus are mixed in manufacturing processes, the manufacturing lines are contaminated, and there are dangers that the products infected with pathogenic microbes such as a virus cause mass infection of final consumers. As the sterilizing methods, heating treatment and treatment with chemicals such as chlorine are exemplified, but these treatments have little effect on viruses having heat resistance and chemical resistance. Therefore, as a means of physically eliminating viruses, membrane filtration using separation membranes has come to attract public attention. According to membrane filtration, viruses can be judged in accordance with sizes and separately removed regardless of the thermal properties and chemical properties of viruses.
As described above, it has been strongly required for separation membranes to have higher water permeating performance, and the improvements to better water permeability have been done repeatedly. For example, there is disclosed in JP-B-1-22003 (the term “JP-B” as used herein refers to an “examined Japanese patent publication”) an asymmetric membrane comprising a polyvinylidene fluoride resin having a structure provided with a thin dense layer for separation on the membrane surface and micro-voids within the membrane for obtaining both excellent separation ability and water permeability. The asymmetric membrane reveals excellent water permeability by the provision of a thin dense layer and micro-voids to thereby reduce filtration resistance. The asymmetric membrane is effective in the case where striking through of a small amount of objective substance to be filtered to the side of permeated water is allowable (e.g., in the case of the removal of turbidity or simple concentration).
However, in the case of eliminating a virus, if the defect such as a pinhole or something like a crack is present on the thin dense layer, the virus strikes through the micro-voids from the defect, so that the virus cannot be eliminated completely. Accordingly, for preventing the striking through of a virus even if a pinhole or a crack occurs, it is required that the separation membrane has a dense layer not containing micro-voids, or even if the dense layer contains micro-voids, the micro-voids are sufficiently small to the thickness of the dense layer.
JP-A-58-91808 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”) and JP-A-58-93734 disclose methods of forming a membrane after adding a nonaqueous alcohol and hydrophilic inorganic fine particles to a solution containing a polyvinylidene fluoride resin, and then extracting and removing these nonaqueous alcohol and the hydrophilic inorganic fine particles to thereby obtain a separation membrane. According to these methods, separation membranes having a dense layer not containing micro-voids can be obtained. However, extraction requires special operations, and if the removal of the additives by extraction is insufficient, these additives remain in the separation membrane as foreign matters. Further, when the thickness of the membrane is reduced to increase the water permeating performance of the separation membrane, it is difficult to reveal physical strength of the suitable degree necessary as the filtration membrane for use in the manufacture of drinking water, water purification treatment, and waste water treatment.
There is disclosed in JP-A-58-91732 the manufacture of a separation membrane by using a liquid containing 20% or more of the solvent component of a polyvinylidene fluoride resin as a solidifying bath to coagulate a polyvinylidene fluoride resin discharged to thereby delay the phase separation rate at the time of solidification and obtain a separation membrane having a dense layer not containing micro-voids. However, according to the method, a void that is intended to be used to form micro-voids is dispersed in the separation membrane at large, as a result, the pore size of the separation membrane as a whole is evened off and becomes large, and the obtained separation membrane has a great surface pore size, so that this separation membrane cannot be used for virus elimination use.
Further, there is disclosed in WO 03/026779 a separation membrane for the elimination of virus comprising a coarse structure layer having a large numerical aperture and a dense structure layer having a small numerical aperture. However, the thickness is as thin as 80 μm or less, and physical strength is sacrificed for revealing practicable water permeating performance, so that it is difficult to use this separation membrane repeatedly as a separation membrane.
In these prior arts, in the case of a separation membrane having formed a dense layer having a surface pore size and a thickness of a degree capable of eliminating viruses and not containing micro-voids, there are drawbacks that when the thickness has practicably high physical strength, water permeability conspicuously lowers and, contrary to this, when water permeability is sufficiently practicable, physical strength is conspicuously deteriorated. Accordingly, it is difficult to reconcile physical strength and water permeating performance on a practicable level.
There is described in the draft of the 29th Annual Meeting of Water Environmental Association Japan, p. 267 (1995) that Qβ of coli phage having a diameter of 23 nm and a spherical structure can be got rid of by 99.99999% or more by membrane filtration treatment with a hollow fiber membrane formed of specific polyacrylonitrile (trade name: Torayfil (registered trademark), article No.: CP10-1010, manufactured by Toray Industries Inc.).
However, since the hollow fiber membrane is a separation membrane made of polyacrylonitrile, low in chemical strength and physical strength, and is deteriorated in a short period of time when membrane filtration operation accompanied by washing with chemicals is performed, so that the membrane is difficult to use practically.
Patent Literature 1: JP-B-1-22003
Patent Literature 2: JP-A-58-91808
Patent Literature 3: JP-A-58-93734
Patent Literature 4: JP-A-58-91732
Patent Literature 5: WO 03/026779
Non-patent Literature 1: The draft of 29th Annual Meeting of Water Environmental Association Japan, p. 267 (1995)